A photodetector is a device that detects the presence or absence of light. A photodiode is a type of photodetector that is commonly formed by combining a p-type material with an n-type material. When exposed to electromagnetic radiation, such as infrared (IR), visible, and ultraviolet (UV) light, a photodiode generates a number of electron-hole pairs at different depths within the device depending upon the depth at which the electromagnetic radiation was absorbed by the device.
Black and white photodiodes are photodiodes that can collect photons over all or substantially the entire visible spectrum, which includes many colors, and therefore generate electron-hole pairs over a large vertical range within the photodiode. On the other hand, wavelength-limited or color photodiodes are photodiodes that collect photons over a much narrower range of wavelengths, such as UV, blue, green, red, or IR, and therefore generate electron-hole pairs over a much smaller vertical range within the photodiode.
Wavelength-limited or color photodiodes are commonly formed as a filtered device by adding a filter to a black and white photodiode. For example, a red wavelength-limited photodiode can be implemented by placing a red filter over a black and white photodiode. The red filter, in turn, only allows red light to enter the photodiode. Thus, although a black and white photodiode is capable of capturing a wider range of the visible light spectrum, the color filter limits the photodiode to collecting only a single color.
Wavelength-limited or color photodiodes are also commonly formed as unfiltered devices by adjusting the depth of the pn junction within the diode. For example, UV light has an absorption depth of less than approximately 0.7 microns, while IR light has an absorption depth of more than approximately 1.2 microns. Thus, an IR photodiode can be formed by utilizing a small depletion region 114 that lies across substantially only the absorption depth of IR light to collect only IR light.
FIG. 1 shows a cross-sectional diagram that illustrates a prior-art IR photodiode 100. As shown in FIG. 1, IR photodiode 100 includes a p-type (boron) region 110, such as a substrate, an n-type region 112, such as an epitaxial layer or well, that overlies and contacts p-type region 110, and a depletion region 114 that is formed across the pn junction between p-type region 110 and n-type region 112.
Photodiode 100 can be formed as a filtered device, e.g., as a black and white device with a large depletion region 114 (that lies across the absorption depths of a number of wavelengths of light), and an IR filter that allows only IR light to enter photodiode 100. Photodiode 100 can alternately be formed as an unfiltered device, e.g., as a device with a small depletion region 114 (that lies substantially only across the absorption depth of IR light) that collects only IR light. Further, photodiode 100 includes an isolation region 116, such as oxide, that is formed on n-type region 112.
In operation, IR photodiode 100 is first reset by placing a reset voltage on n-type region 112 that reverse biases the pn junction. The reverse-biased voltage, which sets up an electric field across the junction, increases the width of depletion region 114 so that the IR portion of the electromagnetic spectrum can be absorbed in depletion region 114.
Once photodiode 100 is reset, photodiode 100 is then exposed to a source of electromagnetic radiation for an integration period. When photodiode 100 is struck by infrared radiation during the integration period, the radiation penetrates into the semiconductor material down to an absorption depth where the IR wavelengths of light are absorbed in depletion region 114.
The IR wavelengths of light absorbed in depletion region 114 generate a number of electron-hole pairs in depletion region 114. The electric field set up across the reverse-biased pn junction attracts the electrons that are formed in depletion region 114 (along with the electrons that are formed in p-type region 110 within a diffusion length of depletion region 114) to n-type region 112 where each additional electron reduces the magnitude of the reset voltage that was placed on n-type region 112.
Thus, at the end of the integration period, the total number of electrons collected by n-type region 112 has reduced the reset voltage to an integrated voltage. As a result, the total number of electrons collected by n-type region 112 during the integration period, which is a measure of the intensity of the received IR electromagnetic radiation, can be determined by subtracting the integrated voltage from the reset voltage. As a result, photodiode 100 can be utilized as a photodetector by indicating the absence of IR light when the difference between the reset voltage and the integrated voltage is small and lies within a range of values, and indicating the presence of IR light when the difference between the reset voltage and the integrated voltage is large and lies outside of the range of values.